1. Field of the Invention
The present invention relates to a metallic container closure having an internal pressure release function, i.e., having a function for automatically releasing the pressure in the container when the pressure in the container is elevated excessively.
2. Description of the Related Art
Usually, a carbonated beverage or the like beverage is filled in a container, and a container closure is mounted on the mouth-and-neck portion of the container to seal the mouth-and-neck portion. When the content in the container is heated to an excess degree in this state, however, the pressure in the container may elevate excessively. The container closure may, further, be once removed from the mouth-and-neck portion of the container and may be mounted again on the mouth-and-neck portion of the container to seal the mouth-and-neck portion. The content in the container, however, may often be rotten and fermented. In this case, too, the pressure in the container may elevate to an excess degree.
When the pressure in the container is elevated as described above, the container closure may jump off the mouth-and-neck portion of the container or, depending upon the cases, the container itself may be broken. To prevent such an inconvenience caused by an increase in the pressure in the container, a metallic container closure having an internal pressure release function has been proposed. As the metallic container closure, there has been known the one in which an internal pressure release line comprising a plurality of slits in the circumferential direction and a breakable narrow bridging portions formed among the slits, are formed at an upper end portion of a cylindrical skirt wall that hangs down from the circumferential edge of a circular top panel wall (see, for example, patent document 1).
With the container closure of the patent document 1, when the pressure in the container is elevated, the bridging portions break, the plurality of slits in the circumferential direction become continuous to form a large slit, the gas in the container is released to the exterior through this portion and, depending upon the cases, the top panel wall is, at the same time, deformed like a dome to release the gas in the container to thereby avoid inconvenience caused by an elevated internal pressure.                patent document 1: Japanese Utility Model Publication No. 7-25318)        
In the above conventional internal pressure releasing metallic container closure, slits directed in the circumferential direction are provided in the upper portion of the skirt wall of a shell of a thin metal sheet, and the internal pressure release line is formed by the slits involving a problem in that deformation takes place from the slits that form the internal pressure release line at the time when the container closure is mounted on the mount-and-neck portion of the container and is wrap-seamed therewith. That is, the container closure is wrap-seamed with the mouth-and-neck portion of the container by putting the shell of a thin metal sheet on the mouth-and-neck portion of the container, pushing the skirt wall of the shell onto the mouth-and-neck portion of the container by using a suitable jig, and transferring the shape of the outer surface (e.g., threaded shape) of the mouth-and-neck portion of the container onto the skirt wall. When the jig is being pushed, however, the skirt wall of the lower portions of the slits is subject to be deformed.
In the conventional internal pressure releasing metallic container closure, further, when the pressure in the container is suddenly and sharply elevated, the bridging portions linking the slits in the circumferential direction are broken over the whole circumference, and an upper portion of the container closure inclusive of the top panel wall is separated away from the skirt wall and jumps out.
Besides, when the shell is made of a thin metal sheet having a tensile strength of about 195 N/mm2, the conventional internal pressure releasing metallic container closure has been so designed that the bridging portions among the slits are broken when the pressure in the container is elevated to release the internal pressure. When the shell is made of a thin metal sheet having a high tensile strength, such as a thin plate of an aluminum base alloy having a tensile strength of 200 to 230 N/mm2, the resistance against drop impact is improved but the bridging portions among the slits are not broken despite the pressure in the container is elevated and the internal pressure is not released. Therefore, the pressure in the container increases to an excess degree still causing such inconveniences that the top panel wall of the container closure jumps out or the container is broken.